Test objects, for instance machine components or the like, are tested after manufacture for the presence of defects such as small cavities or cracks, for example. Such a test is preferably performed using ultrasound. For such ultrasonic testing, the SAFT technique (Synthetic Aperture Focusing Technique) is known for detecting even very small defects inside the test object and for distinguishing said defects from other defects. The SAFT technique can be used to improve the lateral defect resolution, the defect demarcation and the SNR (signal-to-noise ratio) in ultrasonic testing.
Of key importance for assessing the test results is the evaluation of the size of defects, which forms the basis for determining the reliability. For defects that are large compared with the ultrasound wavelength used, measurements can be taken directly from the result of the SAFT analysis. Evaluating small defects, however, is only possible to a limited extent when using the SAFT technique.
In conventional ultrasonic testing, the reference block method and the DGS (Distance-Gain-Size) technique are known methods for evaluating the size of small defects. These methods determine from the maximum echo amplitude what is known as an “equivalent defect size”, which is the size of an idealized reflector that would produce this maximum echo amplitude. In the reference block method, test defects are deliberately introduced at various distances from the measurement surface in a reference block that has the same ultrasonic properties as the test object, and an echo amplitude as a function of the sound path is determined therefrom. As an alternative to this, in the DGS method, the echo amplitude as a function of the sound path can be derived from a DGS diagram supplied by the test-head manufacturer.
Unlike conventional ultrasonic testing, the results of the SAFT technique are amplitude summations, i.e. summations of amplitude values of the ultrasound echoes. Depending on the test object, test grid (i.e. the arrangement of the measurement points on the test object) and test head, these amplitude summations are composed of different numbers of contributory elements, which may include echoes from signals in both central and side regions of the test-head sound beam. Thus the conventional methods for size evaluation of small defects cannot be applied to the SAFT technique.
Langenberg et al: “Imaging And Inverse Scattering In Nondestructive Evaluation With Acoustic And Elastic Waves”, Acoustical Imaging, Nanjung, 12-14 Sep. 1992, Proceedings of the International Symposium on Acoustical Imaging, pages 165-172, discloses numerical modelling for simulating measurements, for example using SAFT analysis. The simulation methods presented are evaluated on the basis of existing measured values.
McGarrity et al: “A facet ensemble approach for evaluation of array performance in ultrasonic NDE”, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Bd. 41, No. 1, 1 Jan. 1994, pages 19-24, ISSN: 0885-3010 discloses a simulation approach for evaluating the performance of arrays in nondestructive testing, in particular using SAFT analysis.
Document EP 2 147 300 A1 describes a modified SAFT technique in which the SAFT amplitude summations are calculated such that they can be compared with the conventional echo amplitude. In this case, however, the defects under inspection must not be too small, so that conventional amplitude evaluation is feasible. On the other hand, however, the defects must be small enough for the reflection not to be direction-dependent. Otherwise the defects would be underestimated.